The majority of the clinical situations suggestive of an increased genetic risk for colorectal cancer (CRC) are not explained by a simple monogenic model. Our working hypothesis is that a fraction of clinical conditions suggestive of an increased genetic risk for CRC (familial aggregation, early age of tumour onset, development of multiple primary CRC) is due to the combination of a limited number of genetic variations, each conferring a moderate risk for CRC, but whose combination results into high risk.